Memristors: A Flash Competitor that Works Like Brain Synapses

In theory, at least, the new technology could allow for a replacement for NAND Flash memory, maybe for DRAM and hard drives, and maybe even for logic at some point.

Last week, I read a lot about HP's advances with
the memristor. This is a new class of tiny switch that could eventually change
some of the fundamental ways computing devices are designed, and I am very intrigued. In theory, at least, the new technology could allow for a
replacement for NAND Flash memory, maybe for DRAM and hard drives, and maybe
even for logic at some point. It's fascinating technology--but of course, the path from theory to commercial product is often longer and more
complex that it initially appears.

Memristors, or memory resistors, were postulated initially by Leon Chua
of the University of California, Berkeley, back in 1961. Essentially,
the idea is that there should be a fourth device, alongside resistors,
capacitors, and inductors. You could put different amounts of electrical
current through the device that corresponded to different states, and the
device would remember that state even after the current disappeared. In
other words, it could function as non-volatile memory. Since then, the
scientific community has periodically discussed memristors, but no big company seemed to take it seriously
until HP announced in 2008 that it had figured out how to actually build
the devices.

Last December, HP published a paper in the "Proceedings of the National
Academies of Science" that detailed a new architecture with which researchers could build a three-dimensional array of memristors and address
each element. Thus, it can store and read large amounts of information,
according to R. Stanley Williams, senior fellow and director of HP's
Information and Quantum Systems Lab.

An image of a circuit with 17 memristors captured by an atomic force microscope. Each memristor is composed of two layers of titanium dioxide sandwiched between two wires. When voltage is applied to the top wire of a memristor, the electrical resistance of the titanium dioxide layers is changed, which can be used as a method to store a bit of data. Credit: R. Stanley Williams, HP Senior Fellow and Director of
Information & Quantum Systems Lab, HP Labs
.
Williams said the devices are manufactured with conventional
lithography: Tiny wires are laid down on silicon and coated in a
very thin layer of titanium dioxide; then a second array of
wires is laid down in a "cross-bar" fashion. Where the wires intersect, they create
memristors. He said that in the lab, researchers have created devices
which have their smallest feature size at 3 nanometers, mainly to
demonstrate that this is a technology that can scale. According to Williams, researchers demonstrated that the devices can switch on and off in about a nanosecond, which is much faster than NAND Flash memory and in the range of DRAM.

Williams said they had tested the devices and proved they could reliably read and write data from one of the cells hundreds of thousands of times, which is better than many forms of Flash memory. Researchers can now have several thousand bits in one array, and the architecture allows the arrays to be linked together. These are important steps in creating a new kind of non-volatile storage.

Memristors Versus Flash Memory

The reason this gets a lot of attention is that many people in the semiconductor industry have been predicting that NAND Flash--by far the most popular current form of non-volatile memory used in computers, portable music players, smartphones, and other devices--may be reaching its limits. Most NAND Flash memory shipped today is produced using 30- to 40-nm processes, with the leading companies saying they can produce products at 20- to 30-nm processes in the next year or so.

Some Flash memory makers, including Samsung, say they believe they can keep scaling NAND technology. Others, such as SanDisk, are talking about new technologies for increasing the numbers of bits per cell they can store. But still others in the memory industry are concerned that the technology will not scale to much smaller geometries.

As a result, a number of memory companies are working on alternatives, such as phase-change memory, which promises the ability to scale to even smaller nodes. That technology involves heat to change a material between a polycrystalline and amorphous state and back, similar to the way data is stored on a rewriteable CD. For instance, Numonyx, originally a joint venture of chipmakers Intel and ST Microelectronics and currently being acquired by Micron Technology, said it shipped phase-change memory samples in late 2008, and has said it would start shipping mass volumes of phase-change memory products this year. But we haven't seen them yet.

Williams said HP is now in the process of transitioning the memristor technology "from lab to fab": in other words, moving it into a manufacturing environment to create prototypes. He said it will take at least 18 months to create and iterate prototypes and then another 18 months to get ramped up to production "if everything goes perfectly."

HP will not be doing its own manufacturing, said Williams; instead, it will be done through partnerships, the typical kind of foundry relationships. "We envision a joint development process where we and a partner work together to get all the bugs worked out and understand what the final product will look like," Williams said.

But he said the manufacturing requires no special technology and no special materials. The first devices are likely to be manufactured using the conventional lithography of the day, and titanium dioxide is a material that has been used in every fab in the world. "Practically anyone could make this tomorrow if they had the design," Williams said.

In about three years, on a 25-nm node with four layers of devices, you could get about 20 gigabytes per square centimeter, he said, noting how the "cross-bar" structure allows for more density. "We anticipate this would be roughly double what Flash memory can deliver at the same time," Williams said. Then noting the creation of 3-nm parts in the lab, he said, "We think this technology can scale a lot farther than Flash memory than."

Williams also said he believes memristor technology is "inherently less expensive" than other forms of memory because, it has a very simple structure that is easy to lay out. He admitted that since other technologies, such as Flash memory, are already in the marketplace, memristors will face "the problem of ramping up and getting to the volumes that would let us take advantage of economies of scale." But he said he was feeling "pretty confident" and "anxious to see how it will play out."

Memristors as Logic

The biggest new news about memristors, though, came in a paper in Nature last week, in which HP announced that the devices can also perform logic functions. In other words, Wiliams said, a memristor can act as both a storage element and a logic element, or "a lock as well as a gate." "There's nothing else I'm aware of that performs both of those functions simultaneously," he said.

Memristors would initially be introduced as a competitor to Flash, he said, and over time they could act as another layer in large data centers as a buffer between DRAM and spinning disks. Still later, memristors could eventually take over much of what DRAM and hard disks do. But in such situations, you would still be shipping data from where it is stored to where it is processed.

Williams said there is an "intriguing possibility" that if you could use the same structure to do actual computing as well as storage, you could send the program to where the data is and execute the problem where the data is stored. Of course, that all depends on what the performance of memristor-based devices ends up being, compared with traditional CPUs and memory systems.

In the really long term, memristor discoverer Chua, who is a professor in the Electrical Engineering and Computer Sciences Department of the University of California at Berkeley, thinks the devices work more like synapses in the brain than conventional semiconductors. "Since our brains are made of memristors, the flood gate is now open for commercialization of computers that would compute like human brains, which is totally different from the von Neumann architecture underpinning all digital computers," he said.

That's because they can actually work as analog devices measuring amounts of currents, not just digital signals that are on or off. (The initial products will just be used as digital devices, Williams said, because it is easier and also what is compatible with today's computers.)

In such a system, we would have what Williams called "synaptic computing," where the system might have the ability to both recognize signals and learn from them, "So you teach, rather than program," Williams said. But such analog computing is very different from what people are used to, and this would happen only in the very long term. In the meantime, HP says it hopes to be able to product a Flash memory competitor in about three years.

This all sounds great, but I'll caution that there's always a long path from lab prototypes of a new technology to actual production, as the long road taken by phase-change memory points out. HP has not yet produced sample parts even for use as memory--only lab prototypes. For this to really matter in the next few years, even assuming it is just for memory, HP has to prove it can be manufactured cost efficiently. Then it and its partners have to commit to actual production, and then make the parts. That whole process is likely to take at least three years.

I'll be very interested to see how far memristor technology comes, along with other alternative memory techniques, in that period. It should make for a fascinating competition.Back to top

Michael J. Miller's Forward Thinking Blog: forwardthinking.pcmag.com
Michael J. Miller is chief information officer at Ziff Brothers Investments, a private investment firm. From 1991 to 2005, Miller was editor-in-chief of PC Magazine, responsible for the editorial direction, quality and presentation of the world's largest computer publication.
Until late 2006, Miller was the Chief Content Officer for Ziff Davis Media, responsible for overseeing the editorial positions of Ziff Davis's magazines, websites, and events. As Editorial Director for Ziff Davis Publishing since 1997, Miller took an active role in...
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